1. Field of the Invention
The present invention relates to a portable, combustion-engined power tool, in particular a setting tool for fastening elements, and including a combustion chamber, a device for feeding fuel into the combustion chamber to obtain therein a combustible air-fuel mixture, and an ignition device for igniting the air-fuel mixture that fills the combustion chamber. The present invention also relates to a method of controlling operation of such power tool.
2. Description of the Prior Art
In a power tool that is formed, e.g., as a nail setting combustion-engined tool, a drive is applied by a piston to a nail-like element for driving it in an object. An ignitable air-fuel gas mixture becomes available when the setting tool is pressed against the object. Upon actuation of a trigger or an actuation lever, an electrical spark is generated that ignites the air-fuel gas mixture, starting the combustion process. The chemically bound energy of the fuel is transformed into the physical energy by the combustion process. The combustion of the air-fuel gas mixture leads to increase of pressure which is transformed via the piston surface, the pressure acts upon, into a linear movement which drives the nail-like element into the object.
Thus, the drive energy for driving in a nail-like element depends from the chemically bound energy available in the combustion chamber and on its transformation in a mechanically usable thermal energy. The latter is determined by a ratio λ of a gaseous fuel to air. This ratio should always be in a region of λ=1 in all of the operational conditions of the power tool. In order to be able to use the power tool in a wide temperature region with wide variations of air density, the mass of the fuel, which is fed into the combustion chamber, should correspond to the air mass which is brought in.
In the presently available setting tool, the feeding of fuel is effected with one or more metering devices which meter a predetermined amount of a liquefied fuel and which are mechanically or electronically controlled. The amount of fuel is determined by a fixed geometry of the metering chamber and by the temperature of the liquefied fuel. Generally, the admixable fuel volume should remain substantially constant over the entire operational region. The injection of the liquid fuel into the combustion chamber is effected in one simple stroke before actuation of the power tool or before the start of the setting process, with a continuous ejection of the fuel from the metering chamber through the metering valve. A new injection begins only after the setting of the nail-like element has been completed.
Controlling the amount of the air mass brought into the combustion chamber to obtain the required ratio λ is very difficult because of the interrupted flow of air during the tool operation. Besides, the air density changes linearly with the air temperature. As a result, the fuel-air ratio λ changes even when the amount of the admixed fuel is constant. With an unfavorable fuel pressure ratios at temperatures at and below 0° C., an entire gas amount is not ejected from the metering chamber(s), which further adversely affects the fuel-air ratio λ. As a result, too little or too much of air mass is available to react with the gaseous fuel which may adversely affect the energy conversion.
Accordingly, the object of the invention is to provide a method of controlling a portable, combustion-engined tool which would insure a most favorable energy conversion at different operational parameters and/or environmental condition.
Another object of the invention is to provide a power tool in which a most favorable energy conversion can be obtained at different operational parameters of the tool and environmental conditions the power tool operates at.